The invention relates to thermally-removable polyurethanes. More particularly, the invention relates to thermally-removable polyurethanes prepared using the Diels-Alder cycloaddition reaction and their method of making.
Polyurethanes are commercially used as solids or foams for a wide variety of applications. Polyurethanes are useful because they can provide flexibility combined with structural integrity, including resistance to solvents and mechanical strength. For example, electronic components can be potted using polyurethanes to protect the component from adverse environments. However, situations occur which require subsequent access to the components, necessitating using a potting material that is easily removable. Current techniques, such as those used for traditional urethane foams, involve labor-intensive mechanical removal, thereby posing a substantial threat for component damage during material removal as well as increased expense for such labor intensive methodologies.
Polyurethanes result from the irreversible, non-equilibrium reaction of diols (compounds with two alcohol functional groups), or more generally, polyols (compounds with at least two alcohol functional groups) with isocyanates (such as diisocyanates, or more generally, polyisocyanates). Contrary to most other step growth reactions, urethane formation does not involve release of a small molecule such as H2O, HCl, CH3OH and other like molecules. Although the reaction is possible in the absence of a catalyst, tertiary amines and metal salts can accelerate the reaction rate significantly.
Two side reactions can occur upon polyurethane synthesis: formation of both allophanate biuret linkages from an isocyanate functionality and an already existing urethane link. Both reactions not only affect stoichiometry, but also introduce branching, yielding cross-linked species. Polyurethanes can thus be formed through the branching and cross-linking.
In forming the polyurethanes, both physical and chemical cross-linking can occur. Diols used in polyurethane synthesis have a very flexible backbone, The urethane link, however, tends to be rigid. In the resulting polymer with these two components, the flexible backbones tend to aggregate, forming what are called soft segments, and the rigid urethane links tend to aggregate to form what are called hard segments. The hard segments do not dissociate easily and thus create physical, rather than chemical, cross-links in the polyurethane material. This provides further mechanical strength and solvent resistance. In order for this material to be thermally processable to remove the polyurethane material, it must be heated up above the melting point of these hard segments. This melting point temperature is typically quite high, generally between approximately 150xc2x0 C. and 200xc2x0 C., causing degradation of the polyurethane chain.
Due to these type of cross-links that are typically present in polyurethanes, the polyurethane materials become difficult to melt process (i.e., process the material at a temperature at which the material flows), and difficult to remove once it is potted. If a polyurethane could be made to thermally break into fragments under mild thermal treatment, and then have the fragments rejoin as the material is cooled down, it would be a thermally reversible polyurethane that would provide a method for easy removal as well as easy melt processing. This type of material could be useful as a removable material for applications where melt processing is desired. In industrial terms, this material would combine the benefits of both a thermoplastic and a thermoset.
Addressing the need for materials that are easier to melt process, Onwumere et al. (U.S. Pat. No. 5,491,210, issued on Feb. 13, 1996) describe polymers having thermally reversible bonds, such a thermally reversible aromatic urethane bonds, adapted to evanesce at an elevated temperature. The reactions forming the polymeric material occur at temperatures from 80-200xc2x0 C. and more generally above approximately 120xc2x0 C. The formed polymers are apparently melt processable generally above approximately 170xc2x0 C.
Patel and Vyas (Patel, H. S., and Vyas, H. S., Eur. Polym. J., 1991, 27(1), 93-96) describe syntheses of poly(urethane-imides) prepared by Diels-Alder reaction to contain both urethane linkages and furan-maleimide Diels-Alder adducts. The materials are made by first synthesizing a bis-furan functionalized compound containing two urethane linkages which is then reacted with bis-maleimides to make the polyurethane. The materials are then converted to a non-reversible polymer through additional reactions which converts the furan-maleimide Diels-Alder adduct to an imide linkage to form the poly(urethane-imide) material.
Laita et al. (Laita, H. Boufi, S., and Gandini, A., Eur. Polym. J., 1997, 33(3), 1203-1211) also describe synthesis of polyurethanes prepared with Diels-Alder adduct linkages, with the linkages present in the branches and not the backbone of the polymer. Laita et al. describe polymeric components functionalized with furans rather than with discrete molecules. Laita et al. note that attempts to return to linear structures by heating were not successful.
Useful would be materials that can be have the advantageous structural characteristics of polyurethane materials but also can be easily and efficiently removed through heating at mild temperatures that do not harm the components which are encapsulated by the polyurethane material.